Subterranean drilling method

ABSTRACT

A method of forming a subterranean cavity communicated with the surface through a drilled hole which includes the steps of drilling a hole from the surface of the ground to the subterranean situs at which the cavity is to be located, suspending an explosive at the lower end of the hole on a flexible suspending member extended downwardly in the hole from the surface, packing the hole around the suspending member with a stemming material, detonating the explosive, and then drilling out the stemming material in the hole to communicate the cavity formed by the explosion with the surface.

United States Patent OTHER REFERENCES Carroll F. Knutson [72] Inventors[22] Filed Jan.20,l970 [45] Patented Nov.30,l97l

METHOD [54] SUBTERRANEAN DRILLING 4 Claims, 5 Drawing Figs.

3,506,069 4/1970 Natland...................... l66/247 Minuut BACKGROUNDOF THE INVENTION The invention described herein was made in the courseof, or under, an agreement with the U.S. Department of the Interior.

l. Field of the Invention This invention relates to earth excavation forthe purpose of geological exploration and/or oil and gas production. Ina more specific, though nonlimiting, aspect, the invention relates todrilling methods useful in conjunction with nuclear energy utilizationin oil and gas production.

2. Brief Description of the Prior Art Nuclear explosives have beenproposed for use in stimulating production of oil and gas fromsubterranean formations. By implanting nuclear devices in tight, thickfomiations and detonating the devices, the broken rock resulting fromthe explosion becomes the new well bore, and it has been conservativelyestimated that a production rate of at least six times that of aconventionally completed oil or gas well can be realized from suchprocedure.

ln the work which has been carried out previously in the field ofnuclear fracturing of subterranean formations, the common practice hasbeen to drill an explosive emplacement hole from the surface to thelocation in the formation to be fractured at which the nuclear devicewill be located at the time of detonation. The device with an attachedfiring cable is then lowered to the detonation situs on a suspensioncable extended downwardly in the emplacement hole. A stemming materialis then packed around the cables in the emplacement hole preparatory todetonating the nuclear device. After the nuclear device has beendetonated to fracture the formation, a reentry hole or well is drilledfrom the surface to the cavity formed by the explosion, and thehydrocarbons are produced from such reentry well.

It will be apparent that the drilling of the described reentry well boreconstitutes a further expense over that which would characterize thismethod of production were it possible to use the device emplacement holdfor production. The drilling of the reentry hole also extends the timerequired to bring the production on stream. Drilling out the stemmingmaterial to clear the emplacement hole for production usage would be arelatively quickly and inexpensively performed operation as compared tothe preparation of an entirely new reentry hole, but has not beenpossible due to the interference which the suspension and firing cablesafford to drilling. Thus, in carrying out the drilling in conventionalfashion using a rotary bit, the cables would quickly wrap around andfoul the bit. Moreover, the cables would likely be damaged beyondfurther usage.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present inventionprovides a method for forming a subterranean cavity communicated withthe surface to facilitate production of fluids from the zone around thecavity, or study of the physical and geologic characteristics of thecavity, the material in the cavity and the earthen zone around thecavity, if such study is an objective. The method specifically permitsnuclear fracturing of a subterranean formation followed by communicationof the fractured zone with the surface through the drilling of but onehole from the surface to the formation.

Broadly described, the method of the invention comprises the steps ofinitially drilling a hole from the surface to the subterranean situs tobe fractured, suspending at the situs to be fractured, an explosiveconnected to the lower end portion of a flexible member extended downthe hole from the surface, closing the hole around the flexible memberwith a stemming material, detonating the explosive, and finally,removing sufficient stemming material from the hole to provide access tothe fractured situs. The removal of the stemming material is carried outwith the flexible member in place, and is preferably accomplished with adrill bit which oscillates, rather than rotates, and is therefore notfouled by the flexible member.

An object of the invention is to reduce the cost of producing oil andgas by nuclear stimulation of subterranean formations.

An additional object of the invention is to reduce the time required toproduce oil and gas from underground cavities produced by a nuclearexplosion.

A further and more specific object is to provide an improved method fordrilling a hole to the situs of an underground nuclear explosion.

Additional objects and advantages of the invention will become apparentas the following detailed description of the invention is read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a schematic illustration ofa nuclear device positioned in a subterranean fonnation by suspendingthe device on a cable extended to the surface through an emplacementhole.

FIG. 2 is a sectional view taken along line 2 2.

FIG. 3 is a schematic illustration of the earthen environmentillustrated in FIG. l as it appears after detonation of the nucleardevice.

FIG. 4 is a perspective view, with parts broken away for clarity ofillustration, showing one type of apparatus used for drilling out theemplacement hole following the detonation of the nuclear device.

FIG. 5 is an enlarged sectional view taken along line 5-5 of FIG. 4 andshowing one type of drill bit useful for drilling out the emplacementhole, and showing the orientation of the bit with respect to thesuspension cable during drilling.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTIONReferring initially to FIG. l, a nuclear device l0 susceptible todetonation to produce an explosion is positioned in the earth l2 at alocation within a hydrocarbon bearing formation. The device 10 isconnected to the lower end of a cable 14, or other flexible suspendingmember. The cable 14 extends to the surface through an emplacement hole16, and has a firing cable 18 passing through the interior thereof fordetonating the nuclear device. The emplacement hole 16 can be drilled byany suitable method, such as those conventionally employed in theproduction of oil and gas, and will be comparable in depth to the boreholes drilled for that purpose. In the use of nuclear devices fornuclear stimulation of production, however, emplacement holes must belarger in diameter to some degree than conventional bore holes drilledto produce oil and gas due to the size of the nuclear devices presentlyused.

The emplacement hole 16 is cased by a suitable casing 20 over a majorportion of its length. After the nuclear device l0 has been lowered tothe desired position, the hole around the cable 14 is packed with astemming material 2l, such as gravel. The stemming material 2l ispreferably a relatively soft material which can be relatively easilyremoved by drilling following the detonation of the nuclear device l0.The cable 14 may be clamped at the surface by a suitable clamp 22, orsuch clamping may be deferred until a later time in the procedure ashereinafter explained.

When the packing of the stemming material 2l around the cable 14 iscompleted, the nuclear device 10 is detonated from the surface bypassing an electrical current through the firing cable 18. The explosionof the nuclear device 10 initially creates a fireball having atemperature of several million degrees. The fireball expands, forming acavity which ultimately becomes sufficiently large that the roof willnot support itself and falls in, filling the cavity with rubble. Theappearance of a cavity 24 and the rubble 26 which has fallen into thecavity are depicted in FIG. 3. Radial fractures 28 extend outwardly fromthe cavity 24 for a distance equivalent to three or four times theradius of the cavity, and a striking potential production rate ofhydrocarbons from the adjacent formation is attained.

To produce hydrocarbons from the situs stimulated by the nuclearexplosion, a production well must be formed to convey the hydrocarbonsto the surface. In accordance with the present invention, the stemmingmaterial 2l is drilled out of the emplacement hole 16 to providecommunication between the cavity 24 and the surface of the ground. Onetype of apparatus which can be used for drilling out the stemmingmaterial 21 is depicted in FIGS. 4 and S.

A drilling rig is designated generally by reference numeral 30. On therig floor 32 a modified rotary table 34 is secured in conventionalfashion. The rotary table 34 includes the usual internal ring gear andpinion (not visible) for rotating the kelly drive 36. The rotary tablehas been modified, however, to permit the ring gear to be oscillatedabout its rotational axis, rather than being constantly rotated in onedirection, as is the practice in conventional drilling, The structuralmodifications used to provide oscillation include a gear 38 secured tothe projecting end of the drive shaft 40 which drives the ring gear ofthe rotary, and a rack 42 engaging the gear 38 and secured to one end ofa piston rod 44. The piston rod 44 is connected to, and movable with, apiston (not visible) located in a cylinder 46 and subjected toalternately acting forces which reciprocate the piston within thecylinder. The force for reciprocating the piston may be developed bysteam, hydraulics, internal combustion, pneumatics or any of the othermethods known to the art and calculated to provide sufficient power tooscillate the drill stem as hereinafter described.

As the kelly drive 36 is oscillated by the oscillating ring gear of therotary table, the kelly 48 in the drill stem is also oscillated and theentire drill stem from kelly to bit undergoes such oscillating movement.lt will be noted in referring to FIG. 4 that the cable 14 is guidedalong one side of the drill string by guide brackets 50 disposed atlongitudinally spaced intervals on the string. The cable 14 can slidethrough the guide brackets 50 as the bit descends in the emplacementhole 16, and the retention of the cable alongside the drill string inthis manner prevents it from whipping during oscillation of the drillstem, and protects it from contact with the casing 20. Near the lowerend of the drill string, the cable 14 is extended through a slot 52 in atubing section S4, and passes through the central bore in the tubing outof the central mud passageway in the drill bit S6. The passageway may beenlarged if necessary to accommodate both the cable 14 and the mud flowrequired for drilling.

At the commencement of the drilling operation, the cable 14 isdisconnected at the surface and extended upwardly through the centralmud passageway in the bit 56, through the lower tubing section 54, andthrough the slot 52. lt is then reconnected or simply clamped in anysuitable manner to the top of the casing 20 and the drilling iscommenced. As the drill string is extended downwardly in the emplacementhole 16, the cable 14 is pressed into the successive guide brackets 50on the string. It may be pointed out that, as an alternative to thedescribed cable threading procedure, the bit 56 and lower tubing section$4 can be constructed as segmented structures each having two segmentshinged together, and openable relative to each other to permit the cableto be inserted as described.

The bit 56 has three teeth 56a, 56b and 56e symmetrically disposed aboutthe oscillational axis of the string, and each having a toe portionwhich is tapered to provide a cutting edge which bites into the stemmingmaterial 21 when the bit oscillates in one direction, and to ride backover the stemming material without cutting when the bit oscillates inthe opposite direction. Because of this action, the alternatingtorsional forces across the drill stem couplings will be unbalanced witha resulting tendency of such couplings to be tightened. lf desired,however, the joints of drill pipe may be pinned to each other withremovable pins to form a mechanically positive lock against unthreadingdue to the oscillating movement.

As the bit 56 bears against the bottom of the hole it is drilling, itoscillates about the cable 14. Frictional wear on the cable at thelocation where it passes along the axis ofthe drill stem is reduced bythe lubricating action of the drilling mud which is circulated down thedrill pipe. Also sharp and abrasive cuttings are pushed away from thecable 14 by the outflowing mud as it leaves the lower end of the bit.The mud is circulated upwardly to the surface through the annulusbetween the casing 20 and drill string in conventional fashion. Althoughthe method of drilling proposed by the present invention does subject toa twisting action the suspension and firing cables required for theemplacement and detonation of the nuclear device, oscillation of thedrill stem is controlled to reduce the oscillatory amplitude to a levelat which the cables are not unduly stressed or damaged. As the depth atwhich the bit is located is increased, the length of free cable iscorrespondingly increased, and the amplitude of the oscillations can beincreased or varied to increase the effectiveness of the cutting actiondeveloped by the bit 56.

The disclosure heretofore has been based on the assumption that cable 14will, after stemming, be located approximately along the central axis ofemplacement hole 16. Under these circumstances, a procedure wherein thecable is passed into the central portion of the bit will be mostfeasible. lt is quite possible, however, that under some circumstancesemplacement hole 16 will have deviated from the vertical. Should thisoccur, cable 14 would not be expected to lie along the central axis ofthe emplacement hole, but would rather be in contact with a side of thehole. In this event, an alternative drilling procedure may be adaptedwherein a drill bit smaller than the internal diameter of casing 20 isused to remove stemming material 2l. ln this procedure, the bit will bepositioned offcenter in casing 20 in a direction away from cable 14.Moreover, the size of the bit will be chosen so that there will besufficient clearance between the bit and the side of casing 20 at whichcable 14 is located to allow free oscillation of the bit withoutentanglement of the cable. The upward end of cable 14 would, ofcourse,be clamped at or near the wellhead. Specific embodiments of the presentinvention have been described to illustrate how the invention may bepracticed by adherence to certain basic principles. These sameprinciples may be utilized, however, to achieve the objects of theinvention when certain undisclosed variations in, and departures from,the precise steps and structures herein described are employed.Practices which involve such variations and departures which are yetencompassed by the basic principles underlying the invention aretherefore deemed to be within the spirit and scope of the inventionexcept as the same may be necessarily limited by the accompanying claimsor reasonable equivalents thereof.

What is claimed is: l. The method of forming a subterranean cavitycommunicated with the surface through a drilled hole comprising:

drilling a hole from the surface of the ground to the subterranean situsat which the cavity is to be located;

suspending an explosive in the hole on a flexible suspending memberextended downwardly in the hole from the surface;

packing the hole around the suspending member with a stemming material;

detonating the explosive;

drilling out the stemming material in the hole to communicate the cavityfonned by the explosion with the surface; and

stationarily holding a point on the suspending member at the surfaceduring the drilling out of the stemming material to prevent thesuspending member from descending in the hole. 2. The method of forminga subterranean cavity communicated with the surface through a drilledhole comprising:

drilling a hole from the surface of the ground to the subterranean stusat which the cavity is to be located;

suspending an explosive in the hole on a flexible suspending memberextended downwardly in the hole from the surface;

packing the hole around the suspending member with a stemming material;

detonating the explosive; and

drilling out the stemming material in the hole by oscillating a drillingbit against the bottom of the hole to communicate the cavity formed bythe explosion with the surface.

3. The method of forming a subterranean cavity communicated with thesurface through a drilled hole comprising:

drilling a hole from the surface of the ground to the subterranean situsat which the cavity is to be located;

suspending an explosive in the hole on a flexible suspending memberextended downwardly in the hole from the surface;

packing the hole around the suspending member with a stemming material;

detonatng the explosive; and

drilling out the stemming material in the hole to communicate the cavityformed by the explosion with the surface, wherein drilling out of thestemming material is accomplished by oscillating a drilling bit in anarcuate path with the flexible suspending member located at the centerof oscillation.

4. The method of forming a subterranean cavity communicated with thesurface through a drilled hole comprising:

drilling a hole from the surface of the ground to the subterranean situsat which the cavity is to be located;

suspending an explosive in the hole on a flexible suspending memberextended downwardly in the hole from the surface;

packing the hole around the suspending member with a stemming material;

detonating the explosive; and

drilling out the stemming material in the hole to communicate the cavityformed by the explosion with the surface, wherein drilling out of thestemming material is accomplished by oscillating a drilling bit in anarcuate path with the flexible suspending member located adjacent to andoutside the path of oscillation.

UNITED STATES PATENT OFFICE 56g CERTIFICATE 0F CORRECTION Patent No.3623555 Dated November 30, 197i Inventor(s) Carroll F. Knutson and BruceG. Bray It 1s certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

In line 5 of Column l of the above-referenced Patent, the phrase "U. S.Atomic Energy Commission and the was omitted in the printing of thePatent by the Patent Office. Lines 4, 5, and 6 of Column 1 should read:

The invention described herein was made in the Course of, or under, anagreement with the U. S. Atomic Energy' Commission and the U. S.Department of the Interior.

Signed and sealed this 9th day of May 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attes ting Officer' ROBERT GOTTSCHALK Commissionerof Patents

1. The method of forming a subterranean cavity communicated with thesurface through a drilled hole comprising: drilling a hole from thesurface of the ground to the subterranean situs at which the cavity isto be located; suspending an explosive in the hole on a flexiblesuspending member extended downwardly in the hole from the surface;packing the hole around the suspending member with a stemming material;detonating the explosive; drilling out the stemming material in the holeto communicate the cavity formed by the explosion with the surface; andstationarily holding a point on the suspending member at the surfaceduring the drilling out of the stemming material to prevent thesuspending member from descending in the hole.
 2. The method of forminga subterranean cavity communicated with the surface through a drilledhole comprising: drilling a hole from the surface of the ground to thesubterranean situs at which the cavity is to be located; suspending anexplosive in the hole on a flexible suspending member extendeddownwardly in the hole from the surface; packing the hole around thesuspending member with a stemming material; detonating the explosive;and drilling out the stemming material in the hole by oscillating adrilling bit against the bottom of the hole to communicate the cavityformed by the explosion with the surface.
 3. The method of forming asubterranean cavity communicated with the surface through a drilled holecomprising: drilling a hole from the surface of the ground to thesubterranean situs at which the cavity is to be located; suspending anexplosive in the hole on a flexible suspending member extendeddownwardly in the hole from the surface; packing the hole around thesuspending member with a stemming material; detonating the explosive;and drilling out the stemming material in the hole to communicate thecavity formed by the explosion with the surface, wherein drilling out ofthe stemming material is accomplished by oscillating a drilling bit inan arcuate path with the flexible suspending member located at thecenter of oscillation.
 4. The method of forming a subterranean cavitycommunicated with the surface through a drilled hole comprising:drilling a hole from the surface of the ground to the subterranean situsat which the cavity is to be located; suspending an explosive in thehole on a flexible suspending member extended downwardly in the holefrom the surface; packing the hole around the suspending member with astemming material; detonating the explosive; and drilling out thestemming material in the hole to communicate the cavity formed by theexplosion with the surface, wherein drilling out of the stemmingmaterial is accomplished by oscillating a drilling bit in an arcuatepath with the flexible suspending member located adjacent to and outsidethe path of oscillation.